Is an Italian SIM card usable in Croatia, or should I simply purchase one there? We will be in Croatia for 8 days.
An Italian SIM card should work in Croatia, yes, but check that the specific SIM and plan you get does. E.g. the TIM SIMs seem to have "roaming caps" on data.
But if you have to buy a new SIM in Croatia, at least it should be easier: no need to register with a passport like you do in Italy.
https://prepaid-data-sim-card.fandom.com/wiki/Italy
https://prepaid-data-sim-card.fandom.com/wiki/Croatia